UNIVERSITY OF CALIFORNIA COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY, CALIFORNIA CIRCULAR 335 August, 1934 ELECTRIC HEAT FOR PROPAGATING AND GROWING PLANTS 1 B. D. MOSES 2 and JAMES R TAVEENETTI 3 For many years hot water, steam, and decomposing manure have been used as sources of heat to stimulate the growth of plants and to protect them against frost. Recently, however, electricity has proved both suc- cessful and practical for this purpose; and, because of certain advan- tages, its use has increased the interest in artificial heat for plant culture, particularly for warming the soil. The method has been used in Europe for the past ten or fifteen years. According to the story told, a. European gardener, having noticed the more rapid growth of grass over an underground power line near his home, successfully tried heat- ing a small bed with electricity. The device was soon adopted by others. In this country it was first tried in a hotbed in the state of Washington in 1925, but its use did not spread rapidly until the development, several years later, of a special heating cable that could be buried in the soil. USES OF ELECTRIC HEAT IN PLANT CULTURE The most successful and general use of electric heat in plant culture is for propagating, both from seeds and from cuttings. In greenhouses, electricity is a simple and inexpensive means of furnishing bottom heat in the benches. Even in heated greenhouses it can sometimes be economi- cally used to maintain rooting media at a higher temperature than the air, or to heat the benches in the early fall and late spring when the main i This is the eleventh of a series of papers reporting results of investigations con- ducted by the California Agricultural Experiment Station in cooperation with the California Committee on the Eelation of Electricity to Agriculture. 2 Associate Professor of Agricultural Engineering and Associate Agricultural Engineer in the Experiment Station. 3 Associate in Agricultural Engineering and Field Engineer of the California Com- mittee on the Relation of Electricity to Agriculture. 2 University of California — Experiment Station heating system is not in operation. In outdoor hotbeds it has the follow- ing advantages over manure: 1. A predetermined amount of heat, which can be turned on or off to suit conditions, is available at all times. In the manure bed, the heating is continuous and decreases from a maximum shortly after installation to no heat after about 5 weeks. 2. Since the temperature can be regulated either automatically or manually by turning the current on or off, it can be increased or de- creased to control plant growth, or the bed may be converted into a coldflame for hardening the plants. In the manure hotbed, the tempera- ture can be regulated only through ventilation, which requires careful attention. 3. Electric heat is clean and odorless. 4. The electric hotbed can be used for a number of years; once it has been installed, the mere closing of a switch puts it into operation. The top soil can be changed without disturbing the electrical equipment. In the manure hotbed, fresh manure must be installed each time the bed is used. The disadvantages of an electric hotbed are as follows : 1. It costs more than a manure bed. This drawback may be offset over a period of years if the manure has to be purchased. 2. Although the equipment is relatively simple, some knowledge of electricity is necessary to select and install it properly. Electric soil heating to force floral and vegetable plants to mature out of season has been tried but is still in the experimental stage. In green- houses the best results have been obtained when it is used in conjunction with air heating. In outdoor frames or in the open field its use is lim- ited — first, because the cost may be rendered excessive by the relatively large area and long heating period; and, second, because a warm air temperature is also necessary for the growth of certain plants. In experiments with gladiolus at Davis, California, heated plots were brought into bloom from 2 to 8 weeks ahead of nonheated plots; tulips and narcissus, however, showed practically no response. With cucum- bers, there was practically no difference in the time of maturity, but production increased from 25 to 75 per cent. All these experiments were conducted in the open field or in muslin-covered frames during the late winter and spring. In small closed boxes or frames and in small greenhouses, electricity has been economically used for protection against frost. In the large frames used for hardening plants, its value is doubtful. In the open field, soil-heating installations should not be depended upon for frost protection. Cm. 335] Electric Heat for Plants HEATING THE SOIL BY DIRECT CONTACT Soil-Heating Cable. — When the soil is heated by direct contact, a spe- cial cable is used. It consists of an electrically insulated resistance wire (fig. 1) enclosed in a flexible metal sheath about 14 inch in diameter. Its resistance is 500 ohms per 1,000 feet. The manufacturers recommend that 60 feet of this cable be used on 110-volt, and 120 feet on 220-volt circuits. These cable lengths carry a Fig. 1. — Three different makes of soil-heating cable: a, No. 19 resistance wire insulated with felted asbestos enclosed in a lead sheath; b, No. 22 resistance wire in- sulated with two wrappings of waxed cotton and a layer of waxed paper, enclosed in a lead sheath; c, No. 19 re- sistance wire insulated with felted asbestos and two layers of varnished cambric, enclosed in a lead sheath. current of 3.7 amperes at either voltage and give a connected load of 400 and 800 watts, respectively. Shorter lengths increase the wattage and so may injure the cable, whereas longer lengths decrease the wat- tage, thus producing less heat. This cable is priced at 5 to 9 cents per foot, according to manufac- turer, the quantity purchased, and whether bought in bulk or in stand- ard lengths ready for installation. Its life has not been determined, but unless mechanically injured it should last for five years or more. Special Heating Unit. — Figure 2 shows a special heating unit for burying in the soil. It consists of lead-covered cable set in grooves on the top side of the members of a wooden frame — an arrangement which simplifies the installation and protects the cable. It is so wired that University of California — Experiment Station Fig. 2. — A hotbed heated with a special unit consisting of lead-covered cable mounted on a wooden frame. In the foreground is a three-heat switch for man- ually controlling the temperature. (Courtesy of Van-Maenner Manufacturing Co., Affton, Mo.) Fig. 3. — Two different types of soil thermostats: a, self-contained; b, remote, with control-box cover removed. three intensities of heat can be obtained by means of a snap switch. The complete units are made in a number of sizes, and the price varies from $12.00 for one to heat a 6 x 6 foot hotbed to $27.50 for one to heat a 6 x 16 foot hotbed. Thermostats. — Although a thermostat increases the initial cost of the installation, its use is recommended because better results will be ob- tained with less attention. Figure 3 shows two types of thermostats suitable for soil heating. The remote type consists of a control box and Oir. 335] Electric Heat for Plants a gas-filled bulb located at the point where the temperature is to be regu- lated. The bulb is connected to a "sylphon" bellows in the box by a capil- lary tube. The thermostats may be obtained with capillary tubes from 18 inches to 12 feet long. The self-contained thermostat consists of a sealed metal box in which Fig. 4. — An outdoor hotbed showing the heating cable in place ready to be covered with soil. A remote-type thermostat and an en- closed fused switch are mounted on the rear wall. Note how the soil is well banked around the sides. (Courtesy of Eockbestos Products Corporation, New Haven, Conn.) e/ectr/c wires /"or £" iA/a// 6' /" so// Settvee/r co6/e o/7c/ tv/re /?e££//?y So// /?eo£//?0 co6/e 4" to 6" so// 7" 6e£*vee/? ca/>/es tY/re /?e/£//?f /" a/>ove co/>/e Ca/>/e 4" /rosr? yyo//s Fig. 5. — Sectional view of a 6 x 6 foot electric hotbed using 60 feet of cable on 110 volts and a remote-type thermostat. (Courtesy of General Electric Co., Schenec- tady, N. Y.) are mounted bimetallic strips, the breaker points, and the temperature adjuster. It is placed in the soil with its top level with the soil surface. Both types of thermostat open and close the circuit with a quick movement, and both can be adjusted for temperature ranges of from 30° to 120° Fahrenheit. The various sizes range from 10 to 25 amperes' capacity for either 110 or 220 volts on alternating current. The tempera- University of California — Experiment Station ture range between closing and opening of circuit varies from 2° to 6°, according to the make. The prices are from $6.00 to $12.00. Hotbeds in the Ground. — In locating the outdoor hotbed, one should take into consideration good drainage, protection from winds, and ac- cess to a source of electricity. Figures 4 and 5 show the details of con- struction. . Thermostat a/?d l/dse 6/oc/c fase 6/ocAs fenc/osed) ■Mo//? ///?e w/res P/a/7 Safety stv/tc/> 'Tc/se 6/ocAs (e/?c/osed) 7~/?er/7?ostot leads to /?eot/,og cad/e W/ri/?g Fig. 6. — Plan and wiring diagram of a 6 x 24 foot hotbed, heated by four circuits in parallel, on 110 volts, all controlled by a single thermostat. Extra care should be taken to make the frame tight and to have the sash fit well in order to minimize heat losses. A frame width of 6 feet is recommended as convenient for fitting standard sash and for placing the soil-heating cable. The length of the frame should be a multiple of 6 feet as each 110-volt circuit of this cable (60 feet) will heat a 6 x 6 foot bed, and each 220-volt circuit (120 feet) a 6 x 12 foot bed. Several circuits may be connected in parallel and controlled by one thermostat (fig. 6), provided that the same temperature is to be maintained in the soil heated by all the circuits and that the current-carrying capacity of the thermostat is not exceeded. The heating cable should be buried to a depth of 4 to 6 inches and should be spaced as shown in figures 5 and 6. In permanent beds, wire netting should be placed about 1 inch above the cable to protect it from injury by tools. To facilitate removal of the cable in temporary beds, the screen may be omitted. Heat insulators such as cinders and straw, placed under the cable to Cm. 335" Electric Heat for Plants I ^ ^ 4£ T :V-w Wtre />ett//7g * Or6/e £* ••••- .-y, /" a&oye cad/e 060 ise 60 £ torn egcnas33xa±aaai j •-'•'/: V :.V:)^fV' ;•'>':•'•> E ^s^ssszs^ssssss^ggss^ssss^^sssi^^^sa a adore 60 ££ 0/77 3 Fig. 7. — Cross section of bench hotbeds, ^4, For propagating plants in the soil directly over the cable or in pots; B, for propagating plants in flats. TABLE 1 Spacing of Soil-heating Cable and Connected Load per Square Foot for Various Lengths and Widths of Benches When Using 60 Feet on a 110 -Volt Circuit- Width of bench, Length of bed, Distance between Number of cables Connected load, feet feet cable, inches across bed watts per square foot 2 15 6 4 13M 2V 2 15 IVi 4 iom 3 10 6 6 13H 3H 10 7 6 11K 2 4 7M 6 8 13M 4H m 6M 8 U54 5 6 or 1Y 2 6 or 1Y 2 10 or 8 13 M or 10% 6 5 or 6 6or7M 12 or 10 13M or 11 * When 120 feet of cable are used on a 220-volt circuit, the length of the bed is doubled. reduce losses to the soil below, are sometimes used, but their value has not been definitely established. Banking earth against the outside of the frame, however, will reduce heat losses through the sides. If the bed is constructed in a heavy soil, 6 to 12 inches of sandy earth should be placed under the cable to improve the drainage. 8 University of California — Experiment Station Hotbeds in Benches. — An electric hotbed in a bench (fig. 7) is similar to the outdoor type. The heating cable is placed about 2 inches above the floor, with a wire netting an inch above it. When seeds are to be planted, the cable is covered with 4 to 6 inches of soil; but when the bench is to be used for rooting cuttings, the depth should be sufficient to allow 2 to 3 inches between the cutting bases and the cable. If flats are used, the wire netting can be omitted, and the bottoms placed within 1 inch of the cable. Table 1 gives the spacing of the cable for various lengths and widths of benches, calculated so that the two ends emerge at the same point and wiring is simplified. Fig. 8. — A four-flat propagating tray. Open heating coils and a thermostat are mounted on the under side of the tray. INDIRECT SOIL HEATING Figure 8 shows a propagating tray of galvanized iron about 2 inches deep in which are mounted open-heating coils and a thermostat. The tray is inverted, and the flats are placed on it. The common commer- cial sizes are the four-flat (30x40 inches) and the one-flat (15x20 inches), selling for about $10.00 and $7.00, respectively. Figure 9 shows a hotbed with an upper compartment for soil and a lower one for the heating unit. This unit consists of an open coil and a thermostat mounted in a metal frame approximately 33 inches square. One heater that has a connected load of about 300 watts on a 110-volt circuit will heat up to 18 square feet of bed. It can be purchased for about $10.00. Figure 10 shows an enclosed box 3x6 feet, with a rack about 6 inches above the floor, on which flats of soil may be placed. Between the rack and the floor are four open-heating coils that may be connected to oper- ate at a load of 400 watts on either 110 or 220-volt circuits. The tem- perature is controlled by a thermostat mounted on the side above the flats. The entire box, with glass cover and electrical equipment, can be purchased for about $25.00. The electrical equipment, including the coils, thermostat, a pilot light, and connections, costs about $12.50. Cm. 335] Electric Heat for Plants Fig. 9. — A hotbed equipped with a portable heating unit placed in a compartment beneath the soil. Fig. 10. — An enclosed box in which plants are grown in flats. Open heating coils are located under the rack, and a thermostat and pilot light are mounted on the side. 10 University of California — Experiment Station FROST PROTECTION Frames and Boxes. — Soil-heating cable has been successfully used in mild climates for frost protection in frames and boxes either in or out of doors. It may be either installed on hooks around the sides (fig. 11) or laid on the ground. The former arrangement is more convenient but re- //eat//?y Cffd/e 0/7 jo/7 surface Mas///) cover ~&f3/re/e£on frame for Ao/d/no cover Cover at toe/zed £0 /oose Sar fVeot/r/g cc/S/e aftac/zec/ to s/'c/cs B Fig. 11. — Cross section of installations for frost protection. A, Outdoor frame with heating cable on the soil surface; B, indoor bench with temporary muslin cover and heating cable around the sides. suits in less even heat distribution. For tightly constructed and well- covered frames, one 60-foot length of cable on a 110-volt circuit (400 watts) or a 120-foot length on a 220-volt circuit (800 watts) should suf- fice for 72 or 144 square feet of frame, respectively, provided the sur- rounding air temperature does not drop below 25° F. The thermostat should be set to maintain an air temperature of 35° to 40° approximately 3 inches above the soil. Greenhouses. — In greenhouses, heating elements should be evenly dis- tributed under the benches. Operators commonly allow about 0.25 watt Oir. 335 Electric Heat for Plants 11 per square foot of wall and roof area for each degree of difference be- tween the inside and outside temperatures. For example, a greenhouse with 1,000 square feet of wall and roof area will require 3,750 watts to maintain a 40° F temperature inside, when the minimum outside tem- perature is 25° (1,000 x 15 x 0.25 = 3,750 watts). Although soil-heating cable may be used for heating the air in green- houses, it is not well adapted for this purpose, because of the low wat- Fig. 12. — Method of splicing soil-heating cable, a, Resistance wires twisted together and ready to be insulated; b, the joint insu- lated and the % inch galvanized iron pipe being slipped over it; c, the pipe in place and the ends soldered to the lead sheath. tage per foot. In southern California a specially built heating element consisting of asbestos-insulated resistance wire enclosed in %-inch con- duit is being used satisfactorily. This element has a resistance of 0.05 ohm per foot and is used in 80 and 160-foot lengths for 110 and 220-volt circuits, respectively. These lengths have a connected load of 3,000 and 6,000 watts, or 37.5 watts per foot. The cost is about 14 cents per foot. A ready-made air heater constructed especially for greenhouses may be purchased for about 10 cents per foot of heating element. It consists of No. 19 Nichrome wire insulated with felted asbestos and enclosed in a flexible copper tubing. A 33-foot length of this heater connected to a 110-volt circuit will give a connected load of 750 watts. WIRING A wiring diagram for connecting two or more circuits to the main line is shown in figure 6. The safety switch opens the main circuit and should be fused to carry the total connected load, allowing 5 amperes for each length of heating cable. Each individual circuit is protected by 6-ampere fuses. By removing fuses, the operator can disconnect any one or any number of circuits when not needed. 12 University of California — Experiment Station The fuse blocks should always be enclosed in a box for protection from both moisture and accidental contact with the exposed live parts. It is also desirable to enclose the control box of the remote-type thermostat when convenient. When connecting the cable to the main circuit, one should cut the lead sheath back at least % inch farther from the end of the resistance wire than the insulation and should take extra care to prevent the sheath from becoming shorted. The exposed insulation around the resistance 11 lit 11 llliflllfl IS • S H iiiip! i|i|fll|ij W' iii iifllliiii n i|i|}|f|i|i| il WM «P ■ _> - fc'^Ef^M&'iht*^^ ' K' Br K it HiS l • pi ' ; • wF*Jot *m&&S SB *• Fig. 13. — Various types of glass substitutes: a, wire screen coated with cellulose acetate; b, muslin impregnated with paraffin; c, cloth netting enclosed between two sheets of cel- lulose acetate; d, untreated muslin. wire should be given a coat of shellac or varnish to keep it from absorb- ing moisture. All connections should be above the soil and should be well insulated. When the cable is placed in the bench or in any other place where it is not already grounded, the lead sheath should be connected with cop- per wire to a water j:>ipe or to a metal stake driven several feet into the ground. The cable should be coiled or uncoiled by rolling as if it were on a drum. It should never be unrolled by being pulled on one end nor made into loops like a rope. Such handling twists the lead sheath and shortens its life. The cable should not be unnecessarily bent. Being easily dam- aged, it should never be removed from the soil with tools; the best Cm. 335] Electric Heat for Plants 13 method for removing is to soften the soil thoroughly by wetting and then to pull the cable up and coil it at the same time. Figure 12 shows a simple method of splicing a cable. The wire should be bared as described in making connections, and the two ends twisted together and insulated. A piece of %-inch galvanized iron pipe can then be slipped back over the joint, and the ends soldered to the lead sheath. The soldering must be done carefully to prevent melting the lead. HOTBED COVERS Covers on hotbeds are intended to protect the plants from the weather and to conserve heat. Although glass is the best material, its initial cost is so high that muslin and various glass substitutes (fig. 13) are being used. These materials have about the same insulating value as glass, and a smaller initial cost, but their light transmissibility is lower. When new, glass transmits 90 to 95 per cent of the light, muslin from 25 to 35 per cent, and the glass substitutes from 50 to 75 per cent. Because dirt sticks to muslin and the wax-impregnated substitutes, their light transmissi- bility decreases with use. When the covers are used for protection only during the night and are removed during the day, the transmission of light is not an important factor; but when the covers must be left on, insufficient light will cause the plants to become weak and spindly. A high transmissibility of light also results in the greatest benefit from the sun's heat. Regarding the life of the various materials, no definite information is available. With reasonable care, glass sash should last for ten years or more. The substitutes last from one to five years, according to the use and care they receive. Glass sash is relatively heavy and inconvenient to handle but will stay in place without weighting or fastening except during very strong winds. Sash made from the substitutes are light and must be weighted or fastened down even in slight winds. The present price of a ready-made 3x6 foot glass sash is between $5.00 and $6.00. Muslin sells at about 10 cents a square yard; the substi- tutes, at 30 cents to $1.00. Lumber for the frames for these materials will cost about 25 cents per sash. u University of California — Experiment Station TEMPERATURES IN ELECTRICALLY HEATED SOIL Two questions are often asked : "What is the temperature of the heat- ing- cable ?" and "How is the heat distributed in the soil ?" The answers depend upon the depth of the cable, the temperature of 6 . 6/oss c/o//? sos/? T/?er/r?osta£ 6e//& //ee/ec/ x 6ec/ 6//?f?e0ted oec/ 69 r~ 46 "F 63 V 4£°F 43 ">F 33 °f Fig. 14. — Average daily minimum soil and air temperatures at va- rious points in covered frame during December, 1932, January and February, 1933. Thermostat set to cut in at 62° F and out at 68° F. Connected load, 6.66 watts per foot of cable. Average minimum air temperature as reported by official weather observer for Davis, 30° F. A/f&s///? corer 7fier/r?o/ne£er ■•:••-• ■/ W *mm- So// surface .6" .:*■■<; a ■ u-s'^s /Co6/e' -/0c//)c> risrrow k. /S" -j \ Thermo/neter rteoteJ 6ed 66 "f 60 'r~ 6/ r~ V/rAcate* 60cMi0i-lTt-iO OliCifOlfflCOSCRO! CD a; CO » ■?&.OTt-n ■2t3 ft O co a, cs >-..9 •-1 o3 -^ •213 ft O M ID W CN IM M ■* CO CO 0.2 h i- E fl <» cd 9 i-H rH